The present disclosure relates to brushless motors and more particularly to rotors of the brushless motors.
For brushless PM motors, the cost of the permanent magnet depends on the shape of the magnet, and is also a function of the amount of finishing operation needed on the magnets. Surface mounted permanent magnets (SPM) with arc shape are desired to obtain lower cogging torque, lower BEMF harmonics and hence less torque ripple in the motor. Arc magnets (ring type or segment type which could be straight segment or skewed segment) need significant grinding to maintain tighter tolerances if the application requires low ripple performances. These magnets have lower material yield and thus the unit cost is higher too.
Interior permanent magnet (IPM) rotor designs with rectangular shaped magnets may thus be desirable to offset scrap cost and increase material yield. However, due to the shape of the magnet, the motor performance in regards to cogging, harmonics, ripple and thus noise are not ideal. Further, the magnets being located inside the rotor core and the traditional methods of cogging and harmonic cancellation through step skew, make the magnets and rotors in particular, difficult to manufacture. Hence, active cancellation of cogging torque and torque ripple is necessary. Moreover, active cancellation of cogging torque requires some voltage budget for cancellation which may negatively impacts the size of the motor.
Accordingly, improved rotor design is desirable to reduce manufacturing cost and improve torque efficiency and magnet retention.